Magnetic resonance (MR) imaging techniques to provide temperature mapping of anatomical objects, such as tissues, organs, etc. can be useful for several clinical purposes, such as tumor ablation via heating or cooling. For example, real-time knowledge of the temperature of a target tissue (i.e., a tissue under examination) provides feedback to a health professional in determining the correct application of heat or cold, either invasively though a probe or externally through high intensity focused ultrasound (HIFU). Additionally, temperature data of a target tissue may be useful in diagnosing disease in respective patients.
Recently, MR imaging using proton resonance frequency shift has been used to measure real-time temperature changes in tissues. Generally, changes in the temperature of a material result in changes in the proton resonance frequency phase. An MR imaging scanner may be used to measure proton resonance shift of a target tissue and then calculate a change in temperature according to the following:
            Δ      ⁢                          ⁢      T        =                  Δ        ⁢                                  ⁢        Φ                    α        ⁢                                  ⁢        2        ⁢                                  ⁢        π        ⁢                  γ          _                ⁢                                  ⁢                  B          o                ⁢        TE              ,where ΔT is the change in the temperature of the target tissue, Δφ is the change in the magnetic flux of the static magnet of the MR scanner, Bo is the strength of the static magnet of the MR scanner, α is a constant equal to 0.01 ppm/° C., γ is the proton resonance frequency of the target tissue, and TE is the transmit echo time of the MR imaging. However, a baseline of the proton resonance frequency phase needs to be established since this approach measures a shift in the proton resonance. Further, there is a need for accompanying methods to account for a baseline drift of MR images over time and to compensate for the drift, for example, by sampling a region of the target tissue that does not have any anticipated temperature changes.
Other MR imaging techniques may also be able to provide MR temperature mapping. For example, MR elastography has recently been developed to measure the stiffness and viscosity of the liver and assist in diagnosing disease in the liver, such as liver fibrosis. Briefly, MR elastography is implemented by coupling an external acoustic transducer to the patient or target anatomical object. An MR scanner is then employed to apply MR imaging to the patient or target anatomical object using a modified phase-contrast gradient-echo imaging sequence. Multiple phase-offset images are obtained during an imaging cycle. The MR scanner processes the images and uses the wavelength visible on the phase-difference images to calculate a shear modulus and, thereby, measure the stiffness and viscosity of the target anatomical object. Health professionals are extending this MR imaging technique to other applications. It would be advantageous to use MR elastography to obtain temperature measurements of an anatomical object as well. This would have the further benefit of allowing additional patient data to be obtained using existing clinical protocols.